The Herschel Space Observatory, a European space telescope for which NASA helped build instruments and process data, has stopped making observations after running out of liquid coolant, as expected.

The European Space Agency (ESA) mission, launched almost four years ago, revealed the universe’s “coolest” secrets by observing the frigid side of planet, star, and galaxy formations.

This view of the Orion Nebula, taken by the Herschel and Spitzer space telescopes, highlights fledgling stars hidden in the gas and dust clouds. // NASA/ESA/JPL-Caltech/IRAM

“Herschel gave us the opportunity to peer into the dark and cold regions of the universe that are invisible to other telescopes,” said John Grunsfeld from NASA headquarters in Washington, D.C. “This successful mission demonstrates how NASA and ESA can work together to tackle unsolved mysteries in astronomy.”

Confirmation that the helium is exhausted came today at the beginning of the spacecraft’s daily communication session with its ground station in Western Australia. Scientists measured clear rise in temperatures in all of Herschel’s instruments.

Dense filaments of gas in this space cloud, called IC5146, are seen clearly in Herschel's infrared view. // ESA/Herschel/SPIRE/PACS/D. Arzoumanian (CEA Saclay)

Herschel launched aboard an Ariane 5 rocket from French Guiana in May 2009. NASA’s Jet Propulsion Laboratory in Pasadena, California, built components for two of Herschel’s three science instruments. NASA also supports the U.S. astronomical community through the agency’s Herschel Science Center, located at the California Institute of Technology’s Infrared Processing and Analysis Center in Pasadena.

Engineers designed Herschel’s detectors to pick up the glow from celestial objects with infrared wavelengths as long as 0.025 inch(625 micrometers), which is 1,000 times longer than what we can see with our eyes. Because heat interferes with these devices, they were chilled to temperatures as low as 456° Fahrenheit (–271° Celsius) using liquid helium. The detectors also were kept cold because the spacecraft’s orbit is around a stable area called the second Lagrange point about 930,000 miles (1.5 million kilometers) from Earth. This location gave Herschel a better view of the universe.

The Large Magellanic Cloud galaxy is captured in this stunning infrared view from Herschel and NASA's Spitzer Space Telescope.

“Herschel has improved our understanding of how new stars and planets form, but has also raised many new questions,” said Paul Goldsmith from NASA’s Jet Propulsion Laboratory in Pasadena. “Astronomers will be following up on Herschel’s discoveries with ground-based and future space-based observatories for years to come.”

The mission will not be making any more observations, but discoveries will continue. Astronomers still are looking over the data, much of which already is public and available through NASA’s Herschel Science Center. The final batch of data will be public in about six months.

“Our goal is to help the U.S. community exploit the nuggets of gold that lie in that data archive,” said Phil Appleton from the science center.

Highlights of the mission include:
• Discovering long, filamentary structures in space, dotted with dense star-making knots of material.
• Detecting definitively, for the first time, oxygen molecules in space, in addition to other never-before-seen molecules. By mapping the molecules in different regions, researchers are learning more about the life cycles of stars and planets and the origins of life.
• Discovering high-speed outflows around central black holes in active galaxies, which may be clearing out surrounding regions and suppressing future star formation.
• Opening new views on extremely distant galaxies and providing new information about their high rates of star formation.
• Following the trail of water molecules from distant galaxies to the clouds of gas between stars to planet-forming solar systems.
• Examining a comet in our solar system and finding evidence that comets could have brought a substantial fraction of water to Earth.
• Together with NASA’s Spitzer Space Telescope, discovering a large asteroid belt around the bright star Vega.

Other findings from the mission include the discovery of some of the youngest stars ever seen in the nearby Orion “cradle” and a peculiar planet-forming disk of material surrounding the star TW Hydrae, indicating planet formation may happen over longer periods of time than expected. Herschel also has shown stars interact with their environment in many surprising ways, including leaving trails as they move through clouds of gas and dust.